Why is Robert Mueller even investigating the presidential transition?

It is clear from the prosecution of Flynn that Mueller views the transition as within the scope of Mueller’s investigation, otherwise there would have been no reason for Mueller to prosecute Flynn. The lying took place before Mueller was appointed and did not concern conversations during the Trump campaign. For lying to the FBI when he did, Flynn could have been prosecuted by DOJ itself.

Yet Mueller’s team took it on themselves to prosecute the case involving a crime (lying) that took place after the new administration was sworn in regarding a conversation during the transition. The public documents regarding Flynn’s plea make clear that the transition is in focus.

If news reports have any credibility (and they may not), then Mueller is investigating the political actions of the Trump transition team during the transition period.

What authority, however, gives Mueller power to investigate the political strategies of the incoming Trump administration long after the election was over? It does not appear that Mueller has that power under the Order appointing him as Special Counsel....

Deputy Attorney General Rod Rosenstein takes the position that Mueller can prosecute any crimes he finds:...

Could Mueller start investigating street crimes, acting as a roving District Attorney across the land? Just because he found a crime, could he prosecute it, even if not within the proper scope of his investigation?

Rosenstein acknowledged the limits of Mueller’s powers:

“If he finds evidence of a crime that is within the scope of what Director Mueller and I have agreed is the appropriate scope of this investigation, then he can,” Rosenstein said. “If it’s something outside that scope he needs to come to the acting attorney general, at this time me, for permission to expand his investigation.”

Has Rosenstein given Mueller the authority to investigate the political strategies of the incoming administration during the transition? If so, that would be news.